lesson 3 bio botany ,botany ,12th ,12th biobotany ppt ,12th botany ppt ,tn text book ,study materials ,12th study materials, Chapter 3 chromosomal basis of inheritance

1. UNIT VII- CHAPTER
3

2. Chapter outline
• Chromosomal theory of Inheritance – chromosome behaviour and gene behaviour
• Linkage - Eye colour in Drosophila and Seed colour in Maize – complete and incomplete
Linkage
• Crossing over, Recombination and Gene mapping – Holliday model recombination, three point
test cross
• Multiple alleles – self sterility in Nicotiana
• Sex determination in plants – Sphaerocarpos, papaya and maize
• Mutation-types, mutagenic agents and their significance - point mutation types.

3.  T. Boveri (1902) - He was responsible for
developing the chromosomal theory of
inheritance.
 W.S. Sutton (1902) - independently recognized a
parallelism (similarity) between the behaviour of
chromosomes and Mendelian factors.
 Sutton and Boveri (1903) - independently
proposed the chromosome theory of inheritance.
Sutton united the knowledge of chromosomal
segregation with Mendelian principles and called
it chromosomal theory of inheritance.
Sutton and Boveri chromosome theory

4. Salient features of the Chromosomal theory of inheritance
Somatic cells of organisms consist of two identical sets of chromosomes.
One set is received from female and the other from male. In this each two
chromosomes constitute homologous pair.
Chromosomes retain their structural uniqueness and individuality
throughout the life cycle of an organism.
 Each chromosome carries specific determiners or Mendelian factors
which are now termed as genes.
 The behaviour of chromosomes during the gamete formation (meiosis)
provides evidence that genes or factors are located on chromosomes

5. Support for chromosomal theory of heredity
• Thomas Hunt Morgan (1910) on
the fruit fly Drosophila
melanogaster (2n=8).
• The gene for red and white eye
colour are present on the X
chromosome only absent in Y
chromosome.
• This study support that genes
located on chromosomes.
• Also an example for sex linkage

6. Comparison between gene and chromosome behaviour
Comparison of chromosome and gene behaviour

7. Gene and chromosome behaviour during cell division
 The alleles of a genotype are found in the
same locus of a homologous chromosome
(A/a).
 In the S phase of meiotic interphase each
chromosome replicates forming two copies of
each allele (AA/aa), one on each chromatid.
 The homologous chromosomes segregate in
anaphase I, thereby separating two different
alleles (AA) and (aa).
 In anaphase II of meiosis, separation of sister
chromatids of homologous chromosomes takes
place. Therefore, each daughter cell (gamete)
carries only a single allele (gene) of a character
(A), (A), (a) and (a).

8. Number of Chromosomes Ophioglossum

9. Linkage
• It is one of the Mendel’s deviation concept against for law of Independent
assortment.
• The tendency of genes to stay together during separation of
chromosomes is called Linkage. Genes do not independently assort out.
• It was reported in Sweet pea (Lathyrus odoratus) by Willium Bateson
and Reginald C. Punnet in 1906.
• Purple flower long pollen X red flower round pollen
PL/PL pl/pl
• F2 progenies did not exhibit in 1:1:1:1 ratio

10. Arrangement of linked and unlinked
genes on chromosome
Linked genes – close together
Unlinked genes or syntenic genes –
sufficiently far apart
 It is differed by recombination
frequency value
 More than 50% - unlinked genes
 Less than 50% -linked genes

11. Cis-Trans arrangement of genes
Coupling or Cis configuration
Two dominant or recessive alleles
present on same chromosome of
homologous , inherit together into same
gamete.
Repulsion or Trans configuration
Two dominant or recessive alleles
present on different but same
homologous chromosome, inherit apart
into different gamete.

14. KINDS OF LINKAGE
Complete linkage
• The genes located very close together
and do not exhibit crossing over.
• Reported in male Drosophila
• C.B Bridges (1919)
Incomplete linkage
• The genes located sufficiently apart and
exhibit some crossing over.
• Reported in maize
• Hutchinson
Linkage groups- corresponds
to haploid set of chromosomes
Name of organism Linkage groups
Mucor 2
Drosophila 4
Sweet pea 7
Neurospora 7
Maize 10

16. Linkage Crossing over
1. The genes
present on
chromosome stay
close together
It leads to separation
of linked genes
2. It involves same
chromosome of
homologous
chromosome
It involves exchange
of segments between
non-sister chromatids
of homologous
chromosome.
3. It reduces new
gene combinations
It increases variability
by forming new gene
combinations. lead to
formation of new
organism.
Difference between linkage and crossing over

17. Structure of Synaptonemal Complex
Mechanism of crossing over

18. Types of Crossing Over
Depending upon the number of chiasmata
formed crossing over may be classified into
three types.
1. Single cross over: Formation of single
chiasma and involves only two
chromatids out of four.
2. Double cross over: Formation of two
chiasmata and involves two or three or
all four strands
3. Multiple cross over: Formation of more
than two chiasmata and crossing over
frequency is extremely low.

19. Importance of Crossing Over
Crossing over occurs in all organisms like bacteria, yeast, fungi,
higher plants and animals.
 Exchange of segments leads to new gene combinations which
plays an important role in evolution.
 Studies of crossing over reveal that genes are arranged linearly
on the chromosomes.
 Genetic maps are made based on the frequency of crossing over.
 Crossing over helps to understand the nature and mechanism of
gene action.
 If a useful new combination is formed it can be used in plant breeding.

20. Recombination
• Formation of new combination of
characters in an organism called
Recombinants
• This process is called
Recombination.

21. The widely accepted model is Holliday’s
hybrid DNA model.
It was first proposed by Robin Holliday(1964)
1. Homologous DNA molecules are paired side by side
with their duplicated copies of DNAs
2. One strand of both DNAs cut in one place by the
enzyme endonuclease.
3. The cut strands cross and join the homologous strands
forming the Holliday structure or Holliday junction.
4. The Holliday junction migrates away from the original
site, a process called branch migration, as a result
heteroduplex region is formed.
5. DNA strands may cut along through the vertical (V) line
or horizontal (H) line.
6. The vertical cut will result in heteroduplexes with
recombinants.
7. The horizontal cut will result in heteroduplex with non
recombinants.
Holliday model showing Recombination

22. RF = Total number of recombinants
Total number of offsprings
= 6+6 x 100
44+6+6+44
= 12 x 100
100
RF = 12%

23. RF = Number of recombinants x 100
Number of off springs
= 1+1 x
100
49+49+49+49+1+1+1+1
= 2 x 100
200
RF = 1%
Recombination frequency observation

24. Genetic mapping
The diagrammatic representation of position of genes and related distances between
the adjacent genes is called genetic mapping.
It is directly proportional to the frequency of recombination between them. It is also called as linkage
map.
The concept of gene mapping was first developed by Morgan’s student Alfred H Sturtevant in 1913.
Map distance: The unit of distance in a genetic map is called a map unit (m.u). One map unit is
equivalent to one percent of crossing over. One map unit is also called a centimorgan (cM) in honour
of T.H. Morgan. 100 centimorgan is equal to one Morgan (M).
Genetic maps can be constructed from a series of test crosses for pairs of genes called two
point crosses. But this is not efficient because double cross over is missed.

25. Three point test cross
A more efficient mapping technique is to construct based on the results of three-
point test cross.

26. Three point test cross
A more efficient mapping technique is to construct based on the results of three-
point test cross.
The analysis of a three-point cross

28. Gene order showing double recombinant
Gene mapping

29. Uses of genetic mapping
� It is used to determine gene order,
identify the locus of a gene and calculate
the distances between genes.
� They are useful in predicting results of
dihybrid and trihybrid crosses.
� It allows the geneticists to understand
the overall genetic complexity of particular

30. Three or more allelic forms of a gene occupy the same locus in a homologous
chromosomes called multiple alleles.
Multiple alleles
Check your Grasp
There may be multiple alleles
within the population, but
individuals have only two of
those alleles. Why?
Characteristics of multiple alleles
• It occupy the same locus in the homologous chromosome, no
crossing over occurs within the alleles of a series.
• Multiple alleles are always responsible for the same character.
• The wild type alleles of a series exhibit dominant character
whereas mutant type will influence dominance or an intermediate
phenotypic effect.
• When any two of the mutant multiple alleles are crossed the
phenotype is always mutant type and not the wild type

31. Self-sterility in Nicotiana
East (1925) observed multiple alleles in Nicotiana which are
responsible for self-incompatibility or self-sterility.

32. Sex determination in plants
 C.E. Allen (1917).
 It is a complex process determined by genes, the environment and hormones.
 About 94% of all flowering plants sexually monomorphic.
 Some 6% of flowering plants dimorphic.
Sex determination in Silene latifolia (Melandrium album)
1. Y chromosome determines maleness
2. X specifies femaleness
3. X and Y show different segments

33. Sex determination in Papaya
Sex determination in papaya Recently researchers in Hawaii discovered sex
chromosomes in Papaya (Carica papaya, 2n=36).

34. Sex Determination in Sphaerocarpos
• First described among bryophytes
• Sphaerocarpos donnellii which has heteromorphic chromosomes.
• The male gametophyte as well
as the female gametophyte is
an haploid organism with 8
chromosome (n=8).
• The diploid sporophyte is
always heterogametic.

35. Sex determination in maize
Zea mays (maize) is an example for monoecious
Two types of inflorescence.
• Tassel- staminate florets
• Ear- pistillate florets
Sex determination in Maize
(Superscript (+) denotes dominant character)

36. Mutation
• Hugo de Vries (1901)
• He has studying on the plant, evening primrose (Oenothera
lamarkiana) and proposed ‘Mutation theory’.
• Two broad types of mutation
• Point mutation
• Chromosomal mutations
• Mutagens or mutagenic agents
• Mutagenesis
The production of mutants through
exposure of mutagens.
• The organism is said to be mutagenized.

40. Mutagenic agents
Muller (1927) was the first to find out physical mutagen in Drosophila.
Physical mutagens:
Temperature, Radiation
 Sharbati Sonora
 Castor Aruna
Chemical mutagens
Mustard gas, nitrous acid,
ethyl and methyl methane sulphonate (EMS and MMS), ethyl urethane,
magnous salt,
formaldehyde, eosin and enthrosine
Comutagens
The compounds which are not having own mutagenic properties but
can enhance the effects of known mutagens are called comutagens.
Example: Ascorbic acid increase the damage caused by hydrogen
peroxide.

41. Changes in chromosome
number

42. Allopolyploidy
• Two or more basic sets of chromosomes
derived from two different species is called
allopolyploidy.
• Allopolyploids are formed between closely
related species only.
• Example: Raphanobrassica
• G.D. Karpechenko (1927) a Russian geneticist,
crossed the radish (Raphanus sativus, 2n=18)
and cabbage (Brassica oleracea, 2n=18) to
produce F1 hybrid which was sterile.

43. Triticale, the successful first man made
cereal.
(i) Tetraploidy: Crosses between diploid
wheat and rye.
(ii) Hexaploidy: Crosses between
tetraploid wheat and rye.
(iii) Octoploidy: Crosses between
hexaploid wheat and rye.
Colchicine, an
alkaloid
extracted from
Colchicum
autumnale

44. Activity: Solve this
When two plants (A and B) belonging to the same genus but different
species are crossed, the F1 hybrid is viable and has more ornate flowers.
Unfortunately, this hybrid is sterile and can only be propagated by vegetative
cuttings. Explain the sterility of the hybrid and what would have to occur for
the sterility of this hybrid to be reversed.

45. Significance of Ploidy
• Many polyploids are more vigorous and more adaptable than diploids.
• Many ornamental plants are autotetraploids and have larger flower and longer
flowering duration than diploids.
• Autopolyploids usually have increase in fresh weight due to more water content.
• Aneuploids are useful to determine the phenotypic effects of loss or gain of
different chromosomes.
• Many angiosperms are allopolyploids and they play a role in an evolution of
plants.

46. Structural changes in chromosome
(Structural chromosomal aberration)

47. Inversion Translocation